A circuitry of a conventional lighting apparatus of high intensity discharge lamp is described with reference to FIGS. 49 to 51.
As shown in FIG. 49, the lighting apparatus comprises a DC power supply 1A, a series connection of transistors (MOS-FET) Q1 and Q2, and a series connection of capacitors C1 and C2 which is connected in parallel with the series connection of transistors Q1 and Q2. The lighting apparatus further comprises a series connection of an inductor L1, an ignitor 2, a high intensity discharge lamp (high-luminance discharge lamp) DL and a resistor R1, which is connected between a connection point of the transistors Q1 and Q2 and another connection point of the capacitors C1 and C2. Another capacitor C3 is connected in parallel with a series connection of the ignitor 2, the discharge lamp DL and the resistor R1. The lighting apparatus further comprises a voltage sensing circuit 4, a current sensing circuit 9 and a control circuit 3.
The DC power supply 1A comprises a diode bridge connection DB serving as a full wave rectifier rectifying AC voltage of commercial power source Vs to DC voltage, a boosting chopper 11 constituted by an inductor L11, a transistor Q11 and a diode D11, and a control circuit 12 sensing an output of the boosting chopper 11 and controlling on and off of the transistor Q11 so as to make the voltage of the output of the boosting chopper 11 be a predetermined value.
The ignitor 2 is constituted by a pulse transformer PT and so on which applies a high pulse voltage to the discharge lamp DL while the discharge lamp DL is not lighted. The voltage sensing circuit 4 senses a voltage value applied to the discharge lamp DL. The current sensing circuit 9 senses a current value flowing to the discharge lamp DL from the resistor R1.
The control circuit 3 controls switching on and off of the transistors Q1 and Q2 as shown in FIGS. 50 and 51 so as to supply an electric power to the discharge lamp DL, in which voltage of the electric power has low frequency of rectangular pulse. The control circuit 3 is constituted by a power sensing circuit 330 which calculates an electric power necessary for lighting the discharge lamp DL from sensing results of the voltage sensing circuit 4 and the current sensing circuit 9, and a driving circuit 33 which drives the transistors Q1 and Q2 corresponding to the calculation result.
While the discharge lamp DL is not lighted, the transistors Q1 and Q2 are alternately switched off with a low frequency, but the transistor Q1 is frequently switched on and off with a high frequency in a term T1 while the transistor Q2 is switched off, and the transistor Q2 is frequently switched on and off with a high frequency while the transistor Q1 is switched off, as shown in FIG. 50. By such switching operations, a rectangular alternating voltage VDL having a low frequency is generated. The alternating voltage VDL, on which alternating voltages VP of several kV outputted from the ignitor 2 are further superimposed, is applied to the discharge lamp DL.
When the discharge lamp DL is lighted, a proper rectangular alternating voltage is supplied to the discharge lamp DL, as shown in FIG. 51 owing to similar switching operations. The high frequency of the switching on and off of the transistors Q1 and Q2 is several tens kHz, and the low frequency of the switching on and off of the transistors Q1 and Q2 is defined by a frequency of a term addition of the terms T1 and T2, which is generally in a range from several tens Hz to several hundreds Hz.
The voltage applied to the discharge lamp DL while the discharge lamp DL is not lighted, however, becomes a half of the voltage E outputted from the DC power supply 1A, as shown in FIG. 50. In the high intensity discharge lamp, it is necessary to apply a no-load voltage of about 250V to 450V to both terminals of the discharge lamp while the discharge lamp is not lighted in order to make the transition from glow discharge to arc discharge smoothly, after starting up of the lighting of the discharge lamp. Thus, the voltage E outputted from the DC power supply 1A must be increased to two-fold of the above-mentioned no-load voltage. It causes the increase of stresses and the upsizing of elements of the lighting apparatus.
Another conventional lighting apparatus of high intensity discharge lamp is described with reference to FIGS. 52 to 54, which is, for example, shown in Japanese patent No. 2948600.
As shown in FIG. 52, the lighting apparatus comprises a DC power source 1, a series connection of transistors (bipolar transistors) Tr1 and Tr2, and a series connection of capacitors C1 and C2 which is connected in parallel with the series connection of the transistors Tr1 and Tr2. The capacitors C1 and C2 have the same capacitance. The lighting apparatus further comprises a series connection of an inductance L1 and a high intensity discharge lamp DL which is connected between a connection point of the transistors Tr1 and Tr2 and another connection point of the capacitors C1 and C2, a capacitor C3 connected in parallel with the discharge lamp DL, and diodes D1 and D2 which are connected back to back with the transistors Tr1 and Tr2. Control signals shown in FIG. 53 are respectively supplied to control terminals of the transistors Tr1 and Tr2. The capacitors C1 and C2 are respectively applied to about a half of the voltage of the DC power source 1 so as to be charged.
In terms T1 and T3 shown in FIG. 53, the transistors Tr1 and Tr2 are alternately switched on and off. Thus, high frequency alternating voltages VP shown in FIG. 54 are applied to the discharge lamp DL. In a term T2, the transistor Tr1 is high frequently switched on and off while the transistor Tr2 is switched off, so that a positive DC voltage VDL is applied to the discharge lamp DL. In a term T4, the transistor Tr2 is high frequently switched on and off while the transistor Tr1 is switched off, so that a negative DC voltage VDL is applied to the discharge lamp DL.
By selecting the frequency of switching on and off of the transistors Tr1 and Tr2 in the high frequency state to be a value near to a resonance frequency of the inductor L1 and the capacitor C3 in no-loaded state, it is possible to generate a high voltage which is sufficient to start up the lighting of the discharge lamp can be generated by the inductor L1 and the capacitor C3.
In such the conventional lighting apparatus, even though a half of the voltage E outputted from the DC power source 1 is applied to the discharge lamp DL, the no-load voltage which is sufficiently higher can be applied to the discharge lamp DL with utilizing resonance of the inductor L1 and the capacitor C3. There, however, is a problem that an excessive voltage might be applied to the discharge lamp DL due to the resonance of the inductor L1 and the capacitor C3. It will be the cause of increasing the withstand voltage of the switching elements. Alternatively, it will be the cause of complexity of structure of the control circuit so as to prevent the increase of the withstand voltage of the elements.